1. Field of the Invention
The present invention relates to a method for continuously producing a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol. More particularly, the present invention is concerned with a method for continuously producing a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol, comprising continuously feeding a cyclic carbonate and an aliphatic monohydric alcohol to a continuous multi-stage distillation column, and continuously effecting a transesterification between the cyclic carbonate and the aliphatic monohydric alcohol in the presence of a transesterification catalyst in the multi-stage distillation column, thereby continuously producing a dialkyl carbonate and a diol, while continuously withdrawing a low boiling point mixture containing the produced dialkyl carbonate in a gaseous form from an upper portion of the multi-stage distillation column and continuously withdrawing a high boiling point mixture containing the produced diol in a liquid form from a lower portion of the multi-stage distillation column, wherein the transesterification is performed under conditions wherein: (a) the reaction pressure is 5xc3x97104 Pa or less, as measured at the inner bottom of the multi-stage distillation column; (b) the reaction temperature is in the range of from xe2x88x9220xc2x0 C. to less than 60xc2x0 C., as measured at the inner bottom of the multi-stage distillation column; and (c) the multi-stage distillation column has an F-factor in the range of from 0.2 to 5.0. By the method of the present invention, continuous production of a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol can be easily performed with high productivity and high selectivity (i.e., a lowering of the occurrence of by-products), without using complicated equipment. Therefore, the method of the present invention is extremely advantageous from the commercial viewpoint.
2. Prior Art
With respect to the method for producing a dialkyl carbonate and a diol by reacting a cyclic carbonate with an aliphatic monohydric alcohol, various proposals have been made. Most of those proposals relate to the development of catalysts for the above reaction. Examples of such catalysts include alkali metals or basic compounds containing alkali metals [see U.S. Pat. No. 3,642,858, Unexamined Japanese Patent Application Laid-Open Specification No. 54-48715 (corresponding to U.S. Pat. No. 4,181,676)], tertiary aliphatic amines [see Unexamined Japanese Patent Application Laid-Open Specification No. 51-122025 (corresponding to U.S. Pat. No. 4,062,884)], thallium compounds [see Unexamined Japanese Patent Application Laid-Open Specification No. 54-48716 (corresponding to U.S. Pat. No. 4,307,032)], tin alkoxides (see Unexamined Japanese Patent Application Laid-Open Specification No. 54-63023), alkoxides of zinc, aluminum and titanium (see Unexamined Japanese Patent Application Laid-Open Specification No. 54-148726), a mixture of a Lewis acid with a nitrogen-containing organic base (see Unexamined Japanese Patent Application Laid-Open Specification No. 55-64550), phosphine compounds (see Unexamined Japanese Patent Application Laid-Open Specification No. 55-64551), quaternary phosphonium salts (see Unexamined Japanese Patent Application Laid-Open Specification No. 56-10144), cyclic amidines [see Unexamined Japanese Patent Application Laid-Open Specification No. 59-106436 (corresponding to U.S. Pat. No. 4,681,967, EP 110629B, and DE 3366133G)], compounds of zirconium, titanium and tin [see Unexamined Japanese Patent Application Laid-Open Specification No. 63-41432 (corresponding to U.S. Pat. No. 4,661,609, EP 255252B1 and DE 3781742G)], a solid, strongly basic anion-exchanger containing a quaternary ammonium group (see Unexamined Japanese Patent Application Laid-Open Specification No. 63-238043), a solid catalyst selected from the group consisting of a tertiary amine- or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a silica impregnated with a silicate of an alkali metal or an alkaline Dearth metal, and a zeolite exchanged with ammonium ion [see Unexamined Japanese Patent Application Laid-Open Specification No. 64-31737 (corresponding to U.S. Pat. No. 4,691,041)], a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound (see U.S. Pat. No. 4,734,518).
With respect to the method for conducting the above-mentioned reaction between a cyclic carbonate and a diol, the below-mentioned four types of methods (1) to (4) have conventionally been proposed. Hereinbelow, explanation is made with respect to such methods (1) to (4), taking as an example the production of dimethyl carbonate and ethylene glycol by the reaction between ethylene carbonate and methanol, which is a representative example of reactions between cyclic carbonates and diols.
(1) A completely batchwise method.
(2) A batchwise method using a reaction vessel provided at an upper portion thereof with a distillation column.
(3) A liquid flow method using a tubular reactor.
(4) A reactive distillation method.
The completely batchwise method (1) is a method in which ethylene carbonate, methanol and a catalyst are fed to an autoclave as a batchwise reaction vessel, and a reaction is performed at a reaction temperature higher than the boiling point of methanol under pressure for a predetermined period of time [see U.S. Pat. No. 3,642,858, Unexamined Japanese Patent Application Laid-Open Specification No. 54-48715 (corresponding to U.S. Pat. No. 4,181,676, EP 1082B and DE 2860078G), Unexamined Japanese Patent Application Laid-Open Specification No. 54-63023, Unexamined Japanese Patent Application Laid-Open Specification No. 54-148726, Unexamined Japanese Patent Application Laid-Open Specification No. 55-64550, Unexamined Japanese Patent Application Laid-Open Specification No. 55-64551 and Unexamined Japanese Patent Application Laid-Open Specification No. 56-10144].
The batchwise method (2), using an apparatus comprising a reaction vessel provided at an upper portion thereof with a distillation column, is a method in which ethylene carbonate, methanol and a catalyst are fed to the reaction vessel, and a reaction is performed by heating the contents of the reaction vessel to a predetermined temperature. In this method, the produced dimethyl carbonate and methanol form a minimum boiling point azeotropic mixture having a boiling point of 63xc2x0 C./760 mmHg. The boiling point of methanol per se is 64.6xc2x0 C./760 mmHg. In this method, the reaction is performed by using an excess amount of methanol in the reaction system, so that the resultant reaction products can be separated into the azeotropic mixture and methanol, due to the difference in boiling point therebetween, by means of the distillation column provided at the upper portion of the reaction vessel. Specifically, a gaseous mixture of dimethyl carbonate and methanol, which is formed in the reaction vessel, is allowed to ascend inside the distillation column, and during the ascending of the gaseous mixture, the gaseous mixture is caused to separate into a gaseous azeotropic mixture and liquid methanol. Then, the gaseous azeotropic mixture is distilled from the top of the distillation column while the liquid methanol flows down to the reaction vessel so as to be recycled to the reaction system in the reaction vessel.
The liquid flow method (3) is a method in which a solution of ethylene carbonate in methanol is continuously fed to a tubular reactor to perform a reaction at a predetermined reaction temperature in the tubular reactor, and the resultant liquid reaction mixture containing the unreacted materials (i.e., ethylene carbonate and methanol) and the reaction products (i.e., dimethyl carbonate and ethylene glycol) is continuously withdrawn through an outlet of the reactor. This method has conventionally been conducted in two different manners in accordance with the two types of catalyst used. That is, one manner consists in passing a mixture of a solution of ethylene carbonate in methanol and a solution of a homogenous catalyst in a solvent through a tubular reactor to perform a reaction, thereby obtaining a reaction mixture, and separating the catalyst from the obtained reaction mixture [see Unexamined Japanese Patent Application Laid-Open Specification No. 63-41432 (corresponding to U.S. Pat. No. 4,661,609, EP 255252B1 and DE 3781742G) and U.S. Pat. No. 4,734,518]. The other manner consists in performing the reaction in a tubular reactor having a heterogeneous catalyst securely placed therein [see Unexamined Japanese Patent Application Laid-Open Specification No. 63-238043 and Unexamined Japanese Patent Application Laid-Open Specification No. 64-31737 (corresponding to U.S. Pat. No. 4,691,041, EP 298167B1 and DE 3781796G)].
The reactive distillation method (4) is a method in which each of ethylene carbonate and methanol is continuously fed to a multi-stage distillation column to perform a reaction in a plurality of stages of the distillation column in the presence of a catalyst, while continuously effecting separation between the produced dimethyl carbonate and the produced ethylene glycol [see Unexamined Japanese Patent Application Laid-Open Specification No. 4-198141, Unexamined Japanese Patent Application Laid-Open Specification No. 4-230243, Unexamined Japanese Patent Application Laid-Open Specification No. 5-213830 (corresponding to DE 4129316A1, U.S. Pat. No. 5,231,212 and EP 530615A3) and Unexamined Japanese Patent Application Laid-Open Specification No. 6-9507 (corresponding to U.S. Pat. No. 5,359,118, EP 569812A1 and DE 4216121A1)].
However, the above-mentioned conventional methods (1) to (4) have their respective problems as described below.
In the case of each of the completely batchwise method (1) and the flow method (3) using a tubular reactor, it is impossible to achieve a higher conversion of ethylene carbonate than the conversion of ethylene carbonate at the equilibrium state of reaction (the latter conversion is dependent on the composition ratio of the feedstocks fed to the reactor and the reaction temperature). For example, in Example 1 of Unexamined Japanese Patent Application Laid-Open Specification No. 63-41432 (corresponding to U.S. Pat. No. 4,661,609, EP 255252B1 and DE 3781742G) which is directed to a continuous flow reaction method using a tubular reactor and wherein the flow reaction is conducted at 130xc2x0 C. using a feedstock mixture having a methanol/ethylene carbonate molar ratio of 4/1, the conversion of ethylene carbonate is only 25%. This means that large amounts of unreacted ethylene carbonate and unreacted methanol, which are contained in the reaction mixture, need to be separated and recovered, which in turn are recycled to the reactor. Actually, in the method disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 64-31737 (corresponding to U.S. Pat. No. 4,691,041, EP 298167B1 and DE 3781796G), various apparatuses are used for the separation, purification, recovery and recycling of the unreacted compounds.
As described below in detail, the batchwise method (2) using a reaction vessel provided at an upper portion thereof with a distillation column has problems in that the reaction must be conducted for a prolonged period of time and, therefore, a large amount of methanol needs to be used for preventing the lowering of the selectivity for the desired products.
In method (2), in order to compensate for the methanol distilled as an azeotropic mixture of the methanol and the produced dimethyl carbonate, the continuous or batchwise addition of supplemental methanol to the reaction vessel is optionally conducted. However, irrespective of whether or not such an addition of supplemental methanol is conducted, the reaction per se is performed only in a batch-type reaction vessel. That is, in this method, the reaction is batchwise performed under reflux for a prolonged period of time as long as 3 to 20 hours.
In this method, the dimethyl carbonate, which is one of the reaction products, is continuously withdrawn out of the reaction system, whereas the ethylene glycol, which is another reaction product, remains together with the unreacted ethylene carbonate in the reaction system containing the catalyst for a long period of time. This long residence time of the ethylene glycol and the ethylene carbonate in the reaction system causes side reactions to thereby produce polyethylene glycols, such as diethylene glycol and triethylene glycol. For preventing the occurrence of such side reactions and the lowering of the selectivity for the desired products, it is necessary to use a large excess of methanol, relative to the amount of the ethylene carbonate which is batchwise fed to the reaction vessel. In fact, in the conventionally proposed methods, the following examples are noted in which a large excess of methanol is used; that is, use is made of methanol in excess amounts (in terms of the number of moles of methanol per mole of ethylene carbonate or propylene carbonate), such as 14 moles (U.S. Pat. No. 3,803,201), 17 moles (Unexamined Japanese Patent Application Laid-Open Specification No. 1-311054), 22 moles [Unexamined Japanese Patent Application Laid-Open Specification No. 51-122025 (corresponding to U.S. Pat. No. 4,062,884 and DE 2615665B)], and 23 moles [Unexamined Japanese Patent Application Laid-Open Specification No. 54-48716 (corresponding to U.S. Pat. No. 4,307,032, EP 1083B and DE 2860142G)].
In the case of the reactive distillation method (4), it is possible to perform a reaction with high conversion, as compared to methods (1), (2) and (3). In fact, it has been reported that, when the reactive distillation is conducted using a large amount of pure methanol (containing no dimethyl carbonate), relative to the amount of ethylene carbonate, i.e., an amount such that the methanol/ethylene carbonate molar ratio is 9 to 10, the conversion of the ethylene carbonate reaches 100% [see Example 1 of Unexamined Japanese Patent Application Laid-Open Specification No. 4-198141 and Example 11 of Unexamined Japanese Patent Application Laid-Open Specification No. 5-213830 (corresponding to U.S. Pat. No. 5,231,212, EP 530615A3 and DE 4129316A1)].
In method (4), the produced dimethyl carbonate is distilled from the distillation column as a low boiling point product together with the unreacted methanol. Dimethyl carbonate and methanol form an azeotropic mixture. Therefore, the separation of the produced dimethyl carbonate from the gaseous reaction mixture distilled from the distillation column is conducted by special separation methods, such as a distillation method conducted under pressure [Unexamined Japanese Patent Application Laid-Open Specification No. 51-108019 (corresponding to DE 2607003B)]. Generally, by this method, dimethyl carbonate containing no methanol can be obtained, whereas methanol can be obtained only in the form of a mixture thereof with dimethyl carbonate. Therefore, it is difficult to obtain pure methanol containing substantially no dimethyl carbonate. For example, in the Examples of the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 51-108019 (corresponding to DE 2607003B), a methanol/dimethyl carbonate mixture (weight ratio: 70/30) is separated by distillation, and pure dimethyl carbonate is obtained as a column bottom product. However, as a product distilled from the column top, only a methanol/dimethyl carbonate mixture (weight ratio: 95/5) is obtained.
As can be seen from the above, for obtaining pure methanol, an additional separation process needs to be conducted. Therefore, from the viewpoint of ease in practicing a commercial scale production of dimethyl carbonate, it has been strongly desired to develop a method in which the methanol/dimethyl carbonate mixture as such can be used as a feedstock instead of pure methanol.
However, heretofore, only a few techniques have been known, in which only a methanol/dimethyl carbonate mixture is used as a feedstock in method (4). For example, there can be mentioned the method described in Example 5 of Unexamined Japanese Patent Application Laid-Open Specification No. 5-213830 (corresponding to U.S. Pat. No. 5,231,212, EP 530615A3 and DE 4129316A1). However, in Example 5 of this Unexamined Japanese Patent Application Laid-Open Specification No. 5-213830, in which a methanol/dimethyl carbonate mixture (weight ratio: 70/30) is used, the conversion of ethylene carbonate is only 62.8% (calculated from the data of the composition of the product mixture obtained at the column bottom). The reason for such poor conversion resides in that, since the reaction between ethylene carbonate and methanol is an equilibrium reaction, the presence of dimethyl carbonate (which is a reaction product of the above reaction) in the reaction system causes a lowering in the conversion of the ethylene carbonate. Therefore, this method has a problem in that the larger the amount of dimethyl carbonate recycled to the reaction system, the longer the reaction time (residence time) required for achieving a desired conversion and the larger the amount of methanol required for achieving a desired conversion.
Accordingly, in the production of a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol by using method (4) and by making use of recycled methanol in the form of a methanol/dimethyl carbonate mixture, for achieving a complete conversion of ethylene carbonate, pure methanol needs to be supplied in addition to the methanol/dimethyl carbonate mixture as in Unexamined Japanese Patent Application Laid-Open Specification No. 6-9507 (corresponding to U.S. Pat. No. 5,359,118, EP 569812A1 and DE 4216121A1).
However, when the methanol/dimethyl carbonate mixture is used in combination with pure methanol, in addition to the main operation for obtaining the desired products, an additional complicated operation to separate the methanol/dimethyl carbonate azeotropic mixture into components thereof for obtaining pure methanol containing substantially no dimethyl carbonate needs to be conducted [for example, such an additional operation needs to be conducted using, in combination, two distillation columns which have different operation pressures (see Unexamined Japanese Patent Application Laid-Open Specification No. 2-212456)]. In fact, in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 6-9507 (corresponding to U.S. Pat. No. 5,359,118, EP 569812A1 and DE 4216121A1), pure methanol is obtained by the above-mentioned additional complicated operation and used.
As mentioned above, in the case of method (4), it is possible to perform a reaction with high conversion, as compared to methods (1), (2) and (3). However, method (4) has a problem in that the productivity has not been satisfactorily high. Further, method (4) also has a problem in that the occurrence of by-products, such as dialkylene glycol and 2-alkoxyethanol, has not been able to be reduced (e.g., when ethylene carbonate and methanol are used as feedstocks, diethylene glycol and methoxyethanol are by-produced).
For solving the above-mentioned problems of method (4), a proposal has been made wherein a mixture of methanol and dimethyl carbonate is reacted with ethylene carbonate so as to cause the conversion of ethylene carbonate to become less than 100% and the unreacted ethylene carbonate is hydrolyzed to form ethylene glycol, thereby producing dimethyl carbonate and high purity ethylene glycol with high productivity while removing the need to recycle the unreacted ethylene carbonate [see WO 97/23445 (corresponding to U.S. Pat. No. 5,847,189 and EP 0889025A1)]. However, this method requires a hydrolysis reactor or the like, in addition to a distillation column. That is, this method requires complicated equipment.
As can be understood from the above, no method has heretofore been proposed for continuously producing a dialkyl carbonate and a diol, each with high productivity and high selectivity (i.e., a lowering of the occurrence of by-products), from a cyclic carbonate and an aliphatic monohydric alcohol by using only simple equipment.
The present inventors have made extensive and intensive studies with a view toward developing a method which is free from the above problems accompanying the prior art. Specifically, the studies have been made in connection with a continuous method which comprises continuously feeding a cyclic carbonate and an aliphatic monohydric alcohol to a continuous multi-stage distillation column, and continuously effecting a transesterification between the cyclic carbonate and the aliphatic monohydric alcohol in the presence of a transesterification catalyst in the multi-stage distillation column, thereby continuously producing a dialkyl carbonate and a diol, while continuously withdrawing a low boiling point mixture containing the produced dialkyl carbonate in a gaseous form from an upper portion of the multi-stage distillation column and continuously withdrawing a high boiling point mixture containing the produced diol in a liquid form from a lower portion of the multi-stage distillation column. As a result, it has unexpectedly been found that, when the transesterification is performed under conditions wherein:
(a) the reaction pressure is 5xc3x97104 Pa or less, as measured at the inner bottom of the multi-stage distillation column,
(b) the reaction temperature is in the range of from xe2x88x9220xc2x0 C. to less than 60xc2x0 C., as measured at the inner bottom of the multi-stage distillation column, and
(c) the multi-stage distillation column has an F-factor in the range of from 0.2 to 5.0, the F-factor being represented by the following formula (1):
F-factor=ug (xcfx81g)xc2xdxe2x80x83xe2x80x83(1)
wherein ug represents the gas velocity (m/s) in the multi-stage distillation column and xcfx81g represents the gas density (kg/m3) in the multi-stage distillation column,
xe2x80x83it becomes possible to achieve the above objective, i.e., to continuously produce a dialkyl carbonate and a diol, each with high productivity and high selectivity, without using complicated equipment, even when the aliphatic monohydric alcohol/cyclic carbonate molar ratio is low. The present invention has been made, based on this novel finding.
Accordingly, it is a primary object of the present invention to provide a novel method for continuously producing a dialkyl carbonate and a diol, each with high productivity and high selectivity (i.e., a lowering of the occurrence of by-products), from an aliphatic monohydric alcohol and a cyclic carbonate without using complicated equipment.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawing.